Marill's oil-filled tail acts much like a life preserver. If you see just its tail bobbing on the water's surface, it's a sure indication that this Pokémon is diving beneath the water to feed on aquatic plants.

Azumarill

1. Immune refers to taking 0.51x damage when attacking or defending.
2. Not Very Effective refers to taking 0.714x damage when attacking or defending.
3. Very Effective refers to taking 1.4x damage when attacking or defending.
4. Base Stamina/Attack/Defense are what this Pokémon start at. Individual values (IVs) add a max of +15 to each.
5. Base Flee Rate is only the base. Additional factors come into play.